In the manufacture of plate glass by flotation, molten glass is poured on a bath of melted metal, generally tin or a predominantly tin alloy, where it forms a continuous ribbon which is then cooled and extracted by means of extracting rollers that channel the ribbon into a so-called drying chamber annealing furnace.
The thickness of the glass ribbon is determined by tractice forces exerted by extracting rollers on the glass ribbon and the possible action of milled edge rolls acting on the upper edges of the glass ribbon.
In the manufacture of this glass, the glass undergoes a drawing and reaches high displacement rates of up to 15 or 30 meters per minute and more, depending on the thickness manufactured and the productivity of the device. Those high displacement rates produce under the glass ribbon a stream of molten metal directed toward the colder outlet end of the bath and described below as a downstream flow.
The molten metal carried along by the ribbon following the downstream flow strikes the outlet wall of the bath and then, by reflection, forms a return flow of cooler molten metal directed upstream and described below as an upstream flow. This upstream flow is especially great between the edges of the glass ribbon and the side walls of the bath.
The colder upstream flow of molten metal mixes with the downstream flow of molten metal producing turbulences and, in zones, high temperature variations, notably, in the drawing zone where the glass is particularly sensitive to temperature variations. Those turbulences are harmful, for they alter the viscosity of the glass and, therefore, drawing is not homogeneous. This results in deformations of the glass ribbon and also a lateral instability of the glass ribbon which is periodically displaced from one side of the bath to the other. This instability can disturb the temperatures in the drying chamber and thereby cause poor annealing of the glass ribbon and breakage.
To solve those problems, it has been proposed in French Pat. No. b 2,254,527 to set up cross barriers under the glass ribbon in order to prevent the upstream flow reflected by the outlet wall of the tank confining the bath of molten metal from mixing with the downstream flow in the drawing zone. In order to do so, a first dam is provided under the glass ribbon at the downstream end of the drawing zone. This dam only lets a part of the downstream flow run under the ribbon and forces the upstream flow to pass along the edges of the ribbon. A second dam is positioned upstream of the first dam and is situated in the region of maximum acceleration of the glass ribbon. This second dam acts in the same way as the first dam letting only a part of the downstream flow run under the glass ribbon and forcing the upstream flow to pass along the edges of the ribbon.
It has also been proposed in French Pat. No. 2,372,122 to combine with a cross barrier permanent baffles situated above the latter which serve to intercept the streams of metal skirting the side walls of the tank.
One of the disadvantages of the devices described above is that the lower part of the downstream flow strikes the cross barriers or dams under the glass ribbon to create undesirable turbulence.
It has been further proposed in French Pat. No. 2,094,030 to curb the upstream flow of metal by using linear induction motors connected to side dams set up in the zone where the glass is already congealed. The linear induction motors generate streams of molten metal running from the middle of the bath to the sides in order to be picked up by the side dams. Although the upstream flow of molten metal is partly curbed, the structure of this patent does not eliminate the lateral instability of the glass ribbon in the bath any better than previous devices. This is due, notably, to the instability of streams of molten tin to which instability must be added the inevitable periodic variations of the width of the glass ribbon which cause disturbances in the temperature pattern of the drying chamber and, consequently, as previously indicated, poor annealing of the glass ribbon and breakage losses.
It is therefore an object of our invention to provide a process and device to regulate upstream flow of cooler molten metal to improve annealing of a glass ribbon and to reduce breakage losses.